1. Field of the Invention
The present invention relates to compositions for removing dissolved oxygen from drilling and completion fluids for use in high temperature subterranean formations.
2. Description of Relevant Art
Completion operations normally include perforating the casing and setting the tubing and pumps prior to, and to facilitate, initiation of production in hydrocarbon recovery operations. The various functions of drill-in, completion and workover fluids include controlling well pressure, preventing the well from blowing out during completion or workover, and preventing the collapse of the well casing due to excessive pressure build-up. The fluid is meant to help control a well without damaging the producing formation or completion components. Specific completion fluid systems are selected to optimize the well completion operation in accordance with the characteristics of a particular geological formation. “Drill-in” drilling fluids, used in drilling through a producing zone of a hydrocarbon bearing subterranean formation, and completion fluids, used in completing or recompleting or working over a well, are typically comprised of clear brines. As used herein, a “producing zone” is understood to be a portion of a hydrocarbon bearing subterranean formation that contains hydrocarbons, and thus a wellbore penetrating such portion of the formation is likely to receive hydrocarbons from the zone for production. A “producing zone” may alternatively be called a “production zone” or a “pay zone.”
Seldom is a regular drilling fluid suitable for completion operations due to its solids content, pH and ionic composition. Drill-in fluids can, in some cases be suitable for both drilling and completion work. Fluids can contain suspended solid matter consisting of particles of many different sizes. Some suspended material will be large enough and heavy enough to settle rapidly to the bottom of a container if a liquid sample is left to stand (the settable solids). Very small particles will settle only very slowly or not at all if the sample is regularly agitated or the particles are colloidal. These small solid particles cause the liquid to appear turbid (i.e., cloudy or hazy). The potential of particle invasion and/or filter cake buildup to damage a formation by reducing permeability in the producing zone has been recognized for many years. If permeability gets damaged, it cannot be 100 percent restored by any means. Loss in permeability means a decrease in anticipated production rates and ultimately in a decrease in production overall.
Thus, the importance of using clear completion and workover fluids to minimize formation damage is now well recognized and the use of clear heavy brines as completion fluids is now widespread. Most such heavy brines used by the oil and gas industry are calcium halide brines, particularly calcium chloride or calcium bromide brines, sodium halide brines, particularly sodium chloride or sodium bromide, or formate brines.
As used herein, the terms “completion fluids” and “completion brines” shall be understood to be synonymous with each other and to include drill-in and workover fluids or brines as well as completion fluids or brines, unless specifically indicated otherwise.
Completion brines often contain dissolved and entrained air which enters the brines as it is circulated through the drill string into the well bore penetrating a subterranean formation. The presence of oxygen from the air in the brines drastically increases the rate of corrosion and deterioration of metal surfaces in the drill string, casing and associated equipment as compared to such fluids which do not contain oxygen. To minimize such corrosion, and the presence of oxygen, completion brines are frequently treated with oxygen scavengers.
Generally, oxygen scavengers used in completion brines are reducing agents that will react out most of the oxygen dissolved in the brine. Common oxygen scavenger chemistries include sulfites, hydrazine, and erythorbate. Sulfites are not generally used in completion brines because the oxidized product, sulfate, can precipitate and lead to other forms of corrosion.
A preferred oxygen scavenger for completion brines is sodium erythorbate, because it reduces the oxygen concentration in a variety of completion brines without causing precipitation seen with sulfites. However, erythorbate tends to decompose at elevated temperatures. At temperatures of about 275° F. and higher, sodium erythorbate in brine decomposes resulting in transformation of the brine from a desired clear and colorless fluid to an undesired dark, brown opaque fluid. This transformation of the brine is troublesome as it gives rise to concerns that the brine may be potentially corrosion-inducing or damaging to the formation. As used herein, “clear and colorless” with respect to brine or completion fluids means that the fluid has an “NTU” (nephelometric turbidity unit) less than about 20. NTU is an American Petroleum Institute accepted unit related to the suspended solids in a brine (higher NTU=more suspended solids), based on how much light is scattered by a sample. The procedure for determining NTU is described in API RP 13J, “Testing of Heavy Brines,” incorporated herein by reference, and is a procedure well known to those of ordinary skill in the art.
Thus, while there are a number of oxygen scavengers for drilling fluids in the marketplace, there continues to be a need for oxygen scavengers having utility in completion brines for use at high temperatures.